35 ILCS 200/10-100
The valuation determined under Section 10-90 shall be used for each year for which application is made and approved under Section 10-95. When any portion of the land is no longer used for airport purposes, the person liable for taxes on that portion of the land shall notify the chief county assessment officer, in writing, of that fact, and shall pay to the county treasurer, by the following September 1, the difference between the taxes paid in each of the 3 preceding years as based on a valuation under Section 10-90 and what those taxes for each of those years would have been when based on the valuation as otherwise permitted by law, together with 5% interest. If this difference is not paid by the following September 1, the amount of that difference shall be considered as delinquent taxes under this Code.
35 ILCS 200/10-100